The last time I tried to learn it was for Phidgets - I had to write some GUI app examples for OS X and used Cocoa, which is of course written in Objective-C. The problem is that while I kind of learned the language (badly), I never really found a good primer into Cocoa, which is the class library for GUI apps on OS X (on the iPhone it's Cocoa Touch). So I never properly learned how to deal with string, numbers, array, dictionaries, etc.
Needless to say, even after one lecture, I've decided that I need to rewrite the OS X Phidgets examples...
Anyways, on to lectures 3, where I'll learn how to properly manage memory. I've always wondered how to properly use retain/release.